This invention relates to multi-channel x-ray detectors of the type used, for example, in computerized tomography imaging systems. More specifically, this invention relates to an improved x-ray detector and a method for using the detector to compensate for the height-dependent sensitivity thereof.
As is known, cross-sectional images of the internal features of an object may be produced by the methods of computerized tomography. In a typical application of those methods, an x-ray source transmits a planar, fan-shaped beam of radiation through the object undergoing examination. Energy transmitted through the object is detected simultaneously in a detector array having, for instance, hundreds of x-ray detector cells. Typically, the position of the x-ray source is changed to irradiate, in the course of imaging, the object from many different intersecting angles. The x-ray detectors function to convert the detected x-ray radiation into electrical signals having magnitudes which are dependent on the intensity of the detected radiation. The electrical signals from each detector cell are amplified and applied along electrical signal paths, commonly referred to as channels, for interpretation in a known way with the aid of a digital computer to produce cross-sectional x-ray images of the internal features of the object.
The xenon gas and the solid-state detectors are among the common detector types used to detect x-rays. In the xenon gas detector, impinging x-ray radiation ionizes xenon gas held under pressure in the detector. The resulting ion-electron pairs are collected on oppositely charged electrode plates. In the solid-state detector, the x-rays excite a scintillator medium to produce optical wavelength radiation which is converted to electrical energy with the aid of photosensors strategically situated in each detector cell. An x-ray detector employing xenon gas is disclosed in U.S. Pat. No. Re. 30,644 assigned to the same assignee as the present invention. U.S. Pat. No. 4,220,860 discloses an x-ray detector which uses a cadmium tungstate scintillator crystal. Both of the aforementioned patents are incorporated herein by reference. It should be noted that, regardless of the type of detector used, the detection of the impinging radiation must be as free of errors as possible. It is necessary, in order to obtain high-quality images, for example, in computerized tomography, that the gain of each detector cell remains stable for the duration of the calibration period. This time period may be several days or weeks.
It has been found by the Applicants herein, however, that detector cell sensitivity is height dependent and accounts for undesirable ring-like features occurring in reconstructed images. That is, Applicants have discovered that detector cell output changes as the position of the x-ray fan beam on the radiation-receiving aperture (window) of the cell changes in a direction orthogonal to the plane of the beam. Such changes in x-ray beam position occur due to thermally induced movement of the focal spot of the x-ray source, therby producing a corresponding movement of the beam in the detector window. Moreover, it has been found that detector cell sensitivity to beam height variations is not identical from cell to cell. As a result, if calibration and actual patient scans are generated at different x-ray tube thermal states, the beam position on the detector windows will be different, and calibration errors will accrue. These errors produce image-degrading errors (artifacts) in the reconstructed images. It is noted that for a typical variation in sensitivity of 0.5%/mm for xenon gas detector cells, a change in the position of the fan beam of only 0.02 mm is sufficient to generate significant ring-shaped artifacts near the center of the image for a rotating detector scan geometry.
The problem could be solved if either the focal spot could be forced to be stationary, or the detector sensitivity could be made uniform. Current state of the art of xenon detectors appears to preclude significant improvement. The height-dependent sensitivity may be even more pronounced with detector arrays using scintillator crystals for which sensitivities of 5%/mm have been measured. Likewise, the thermal stability of high-power (tens of kilowatts) x-ray tubes is not likely to improve significantly.
It is, therefore, an object of the invention to provide an improved detector array having means for measuring the position of an x-ray beam on the aperture of a detector cell, which measurement is useful in compensating for the height-dependent detector sensitivity.
It is another object of the invention to provide a method for compensating for height-and-channel-dependent detector sensitivities so as to reduce image artifacts produced thereby.